The present invention relates generally to seals designed to prevent the leakage of fluid between mating surfaces. More particularly, the present invention is directed to elastomer energized seal assemblies for use in both high and low pressure hydraulic systems.
The adequate sealing of hydraulic devices to prevent fluid leakage is of major importance to the design, construction and operation of hydraulic equipment. Adequate seals and sealing assemblies are important in providing proper operation of high and low pressure hydraulic devices such as landing gear struts, hydraulic power tools, hydraulic actuators, valves, swivel glands and any other hydraulic devices of apparatus.
The sealing of hydraulic devices basically involve preventing hydraulic fluid from leaking between two or more mating surfaces. The mating surfaces may be relatively reciprocal as in landing gear struts or hydraulic power tools. For static hydraulic devices, such as hydraulic actuators, the mating surfaces may remain static. Further, the mating surfaces may rotate or oscillate relative each other with or without longitudinal movement.
The mating surfaces are typically cylindrical surfaces with numerous other surface configurations being possible. The mating surfaces are separated by a small gap to allow movement of the two surfaces relative each other during operation or initial assembly. In general, a seal assembly is mounted within a groove in one of the surfaces and extends annularly across the gap between the two surfaces for sealing against the other surface.
The cross section of a seal assembly commonly used for hydraulic seals is shown generally in FIG. 5 at 9. The seal 9 is commonly referred to as an elastomer energized seal. The elastomer energized seal 9 includes a plastic cap or sealing ring 10 which is energized against the mating surface to be sealed 18 by an elastomer expander ring 12. Expander rings, such as expander ring 12, are also commonly referred to as "pressure rings" or "support rings".
As with other conventional O-ring type seals, the elastomer energized seal assembly 9 is mounted within groove 14 in the other mating surface 16. The seal assembly 9 is designed to prevent leakage of hydraulic fluid through gap 20 between the mating surfaces 16 and 18. The seal assembly 9 includes three leak surfaces or gaps through which hydraulic fluid may leak past the seal. The first leakage surface is between the expander ring 12 and the bottom of groove 14 in mating surface 16. The second leak surface is between the expander ring 12 and sealing ring 10. The third leak surface is between the sealing ring 10 and the mating surface 18.
The cap ring 10 shown in FIG. 5 is conventionally made from a plastic material, such as TEFLON (polytetrafluoroethylene). The expander ring 12 is typically made from elastic materials such as natural rubber and synthetic equivalents having similar elastic properties.
When the hydraulic fluid in gap 20 is under high pressure, the expander ring 12 is energized to force sealing ring 10 against mating surface 18. In this high pressure environment, little if any hydraulic fluid leaks past the sealing ring 10 or other leak surfaces. However, in low pressure hydraulic devices and during the low pressure mode or cycle of high pressure devices, the expander ring 12 is no longer energized and leakage of hydraulic fluid may occur. At these low pressures, the amount of fluid leakage past the seal is dependent upon the amount of pressure which the expander ring 12 exerts against the sealing ring 10 and the other leak surfaces. The pressure exerted by the expander ring is determined by the particular properties of the elastomer being used and the design of the expander.
Teflon is preferred as a sealing ring material since it provides a good seal under high pressures while functioning as a solid lubricant. However, due to its inherent in elasticity, the TEFLON must be continually pressurized against the surface to be sealed to prevent leakage when low pressures are present in the hydraulic device. If the mating surfaces are moved relative each other in such a low hydraulic pressure environment, a thin film of hydraulic fluid will be allowed to pass between the sealing ring and the surface against which it is forced. The thickness of hydraulic fluid film will be dependent upon the amount of unit loading or pressurization provided by the expander ring 12 against the sealing ring 10. As will be realized, the amount of leakage of hydraulic fluid past seal assembly 9 will be directly related in part to the thickness of this film which is allowed to pass by the seal assembly.
Low pressure leakage of hydraulic fluid past seal assemblies such as the one shown in FIG. 5 is also a problem for rotary, oscillatory and static hydraulic environments. Even without longitudinal movement of the mating surfaces relative each other, hydraulic fluid will tend to leak past the seal assembly during low pressure conditions due to the decreased pressurization of the sealing ring against the mating surface as well as the other two leak surfaces.
For expander rings made from the same elastic material, the amount of pressurization which can be obtained from the expander ring and the distribution of the pressure depends to a large extent upon the expander ring shape. The expander ring 12 shown in FIG. 5 provides pressure application across the entire cap ring or sealing ring 10. The amount of sealing ring energization available from such a expander ring design is limited. Increases in sealing ring pressurization can be achieved by increasing the amount of elastic material in the expander ring 12; however, the amount of elastic material which can be compressed into groove 14 is limited.
In order to provide higher pressure support to cap rings during low hydraulic pressure operation, an expander ring such as the one shown at 22 at FIG. 6 was developed. The expander ring 22 includes pressure ridges 24 and 26 which extend annularly around the expander ring 22. These pressure ridges are designed to provide localized increased pressure or unit loading annularly around the cap ring to reduce low pressure fluid leakage past the seal.
Although the expander ring shown in FIG. 6 has provided seal ring pressurization at low hydraulic pressures, the expander ring does not provide high enough unit loading to prevent entirely fluid leakage from occurring at low hydraulic pressures even though the total force may be equivalent to that provided by the expander shown in FIG. 2. Accordingly, there is a continuing need to provide an elastomer energized seal assembly in which the expander ring provides maximum pressurization of the cap ring at low hydraulic pressures to reduce fluid leakage to even lower levels.